I've been coaching youth bowlers now for more than 15 years (and coaching bowling more than 25, which is half my life now) and I've heard the theory about their hands still growing and damaging them. I've also spoken with my orthopedic surgeon and the physical therapy group they work with on this. My belief, after researching as well as from my personal experience, is there is no basis in fact of the fear of putting a youth bowler of any age in a finger tip grip.
My coaching philosophy is as soon as they can repeat a 4 or 5 step delivery, be it 6 years old or 14 years old, and the key word is repeat, they are ready to stretch their hand. I also have taken the same approach as a parent with my 10 year old daughter. As soon as we had her approach down, I, ONLY with my daughers agreement, had the ball move from conventional to fingertip. I did not go up in weight when I first stretched her hand. She was 8 1/2 when I changed her grip, and is now 10 and throwing a 10 pound ball with out any issues. She also hooks the lane but when she executes properly, has a very soft release, sand understands a good bit about lane play (could that be from being with her dad the coach so much, I suspect so).
I have also had 15 year olds who could not repeat their approach and I would not advocate them getting a fingertip grip.
If you have a conventional grip, you have to squeeze the ball to a certain degree to have it stay on your hand. A correctly drilled fingertip ball needs no pressure. The earlier in age we can begin teaching the good habits of not squeezing the ball, the easier it will be for them to stop and stay stopped with newer equipment. Once the fingertip drilling is in place you must check the fit often. You can't outcoach a bad fit, as we all know.
Also, I personally do not start them out with grips in the fingers. Just holes in the ball.
Hope this helps and let me know if you have any questions.
BTM Youth Writer